EMRFD Message Archive 10289

Message Date From Subject
10289 2014-09-23 17:16:20 Chris Howard w0ep A materials question - pcb as capacitor
I am building a small transmitting loop antenna for VHF.
It is made of 1/2" copper plumbing pipe in an octagon shape
with feed on one side using a gamma-match type arrangement.
On the opposite side from the feed is a tuning capacitor.

The antenna is for fixed frequency use.

I used the magnetic loop antenna calculator at 66pacific.com
to create a specification.

According to that I need a tuning capacitance of ~ 12 pf
Capacitor voltage of 1,311 volts RMS
resonant circulating current of 9.78 Amps

I was thinking of having a piece of pcb soldered between
two pipe caps act as my capacitor. I would then trim and
file down the size of the pcb to adjust my capacitance
to the value necessary to resonate at the design frequency.
I figured I would start with about 2 square inches and keep
trimming until I had it where I want it.

The problem that I have is that I'm not so sure that
the pcb material I have (pedestrian stuff from an ebay
purchase years ago) will handle 1,311 volts.

Am I asking for trouble?

Chris
w0ep
10290 2014-09-23 17:36:19 Todd F. Carney / ... Re: A materials question - pcb as capacitor
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10291 2014-09-23 17:37:29 Dana Myers Re: A materials question - pcb as capacitor
10292 2014-09-23 18:41:35 Nick Kennedy Re: A materials question - pcb as capacitor
I tried PCB material for a loop tuning capacitor a few years ago.  

The problem I had was this:  even at 5 watts, there was enough self-heating of the capacitor to change its value and make the resonant frequency change and the SWR skyrocket.  BW was only a few kHz so it didn't take much.

I was trying to do this on 40 M with a 10 foot circumference loop, so that's quite a challenge.  Maybe on 30 M or 20 M things would have been better.

This was the typical glass epoxy type stuff.  Maybe if you could get hold of some Teflon dielectric board ...

73-

Nick, WA5BDU
10293 2014-09-23 20:05:54 ad7jt Re: A materials question - pcb as capacitor

 

 

10294 2014-09-23 20:06:31 ad7jt Re: A materials question - pcb as capacitor

 

 

10295 2014-09-23 20:27:01 Eamon Egan Re: A materials question - pcb as capacitor

Agreeing with a few previous posts, I would say breakdown through the FR4 is not likely to be a problem. However with this voltage I would wonder whether, without removing copper around the periphery, it could arc at the edges.

 

And as also remarked in another previous post, power dissipation is more of a concern. Here I try to put some values to it.

 

This page http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm gives a loss tangent of 0.008 for FR4 at both 100MHz and 3GHz.

 

This page http://www.epectec.com/pcb/laminate/ shows loss tangents in the same ballpark at 1MHz.

 

You don’t say what frequency it’s for, but if we take 10MHz as a typical HF frequency, with 12pF, XC= 1326 ohm.

 

If I properly understand the meaning of loss tangent, it is the proportionality between the capacitive reactance and the ESR. So for a loss tangent around 0.01, you would have an ESR of 13 ohms, which with a circulating current of 9.78A would dissipate over a kilowatt.

 

The practical upshot is that you would not generate this circulating current, as the circuit would not have a high enough Q.

 

I would appreciate it if anyone can correct me if I’ve misunderstood or miscalculated anything.

 

73

Eamon

VE2EGN or AB1NK

10297 2014-09-23 23:58:07 Clutter A materials question - pcb as capacitor
As others have suggested, teflon would be a way better choice of dielectric.
I wouldn't do this particular job with G-10, FR-4 etc. on a bet! I don't see
fiberglass epoxy holding up well to this sort of extreme voltage and current
at HF radio frequencies or above. Even for QRP you are pushing it hard IMO.

Air is a pretty good dielectric choice in this application- perhaps adjustable plate
spacing might be workable? You only need 10pF. I'd make it pretty wide spaced,
and of course cover to protect from rain. I've seen some clever arrangements
on line where copper sheet is hinged at one edge only and opens/closes like the
covers of a book.

Even polyethylene might be better (saw apart a plastic cutting board or whatever)
Ditto for Lexan polycarbonate, Plexiglass, and such. Definitely better IMO.
Make it pretty thick, though- despite whatever high breakdown volts per mil
it might claim...

I like flashing copper, as heavy as possible, silver soldered to your copper pipe.
I agree with the previous suggestion that epoxy laminate with copper right up
to the edges would be at very high risk of flashover. It would probably be of benefit
to have the insulation stick out a ways on all sides, regardless of insulation.

73, David K3KY
10298 2014-09-24 11:15:11 bob_ledoux Re: A materials question - pcb as capacitor
Tube transmitters often used a plate neutralizing capacitor that consisted of a round fixed plate and a second plate on the end of a screw.  Turning the screw varied the capacitance by changing the plate to plate distance.

A similar technique might work here.
10299 2014-09-24 11:42:25 Gustavo V Re: A materials question - pcb as capacitor
Chris, 

use two pieces of copper to make your capacitor and tune it changing the gape between sheets. 

look the last picture for an example:




73's
Gustavo
LU6AVM

10300 2014-09-24 12:41:41 William Martin Re: A materials question - pcb as capacitor
All,

Could you use a length of coax and tune it by pulling the center conductor in and out?  

Bill. N7EU



    Sent from Bill's iPad 3    


10301 2014-09-24 12:50:51 Dana Myers Re: A materials question - pcb as capacitor
10302 2014-09-24 13:06:14 Chris Howard w0ep Re: A materials question - pcb as capacitor
I think I am going to contemplate for awhile and may
end up tossing this plan and going to a big wheel antenna
or something like that.

First, the thing is at VHF and the whole antenna is about
the size of a dinner plate. It is octagonal, about 3 inches
of 1/2" copper pipe between 45-degree elbow fittings.

I originally built it with a coaxial
capacitor made of 1/4" copper water pipe inside of
1/2" copper water pipe with some teflon tape between.
That actually worked, but it did not have enough mechanical
stiffness to be able to set it to one frequency and expect
it to stay there. I was getting excursions of a MHz or
more with movement of 1/16" of the capacitor.

So I was thinking that something stiff like pcb would
give me mechanical stability with the ability to trim it
to my design frequency since both "arms" would be soldered
to the pcb, one on each side.

But I don't like the pcb idea. So now maybe I make
another sandwich out of something else, or... I don't know what.

It seems like a lot of my challenges in ham radio come
down to mechanical design.

I appreciate all the good input!

Chris
w0ep





10303 2014-09-24 13:22:09 Gustavo V Re: A materials question - pcb as capacitor

All,

Could you use a length of coax and tune it by pulling the center conductor in and out?  


RG8 (not foam dielectric) have 100pF x meter (30pF /feet) 
10304 2014-09-24 13:23:13 Gustavo V Re: A materials question - pcb as capacitor
Chris, 

I can't found it again, but I remember a tunning capacitor with two pieces of PCB in sandwich. 

for HV, just etch a bigger PCB and left 2cm/1" border without copper to avoid sparks...

even can play with the separation between copper to change little the capacity. 






73's
Gustavo
LU6AVM

10305 2014-09-24 13:43:45 David Re: A materials question - pcb as capacitor
On Wed, 24 Sep 2014 12:50:48 -0700, you wrote:

>
10306 2014-09-24 14:23:12 dx11 Re: A materials question - pcb as capacitor

Yes,

Google for “A Tuneable Magnetic Loop Antenna for 7MHz made from Coaxial Cable”, a CARC Buildathon Project by G4FYY July 2008.

I downloaded it a couple of days ago, but can’t remember where.

It is being done somewhat different there, but it is an all coax solution.

 

Cor  PA4Q

 

All,

 

Could you use a length of coax and tune it by pulling the center conductor in and out?  

 

Bill. N7EU

 

10307 2014-09-24 16:41:39 kerrypwr Re: A materials question - pcb as capacitor
Although the PCB-capacitor idea appears to be dead I thought it worthwhile to quote from Wes' Two Faces Of Q paper;

"Two different pieces of PCB material were evaluated.  The first was some standard FR-4, 3.5 x 3.7 inches, 234 pF.  Qc = 47 at 7.1 MHz..  The second piece was 439 pF with a piece that measured 4.5 x 6 inches, with Qc = 1368 at 5.2 MHz.  This second piece was a material called Duroid ...".

There are quite a few kinds of Rogers Duroid; most have dissipation factors in the approximate range of 0.001 to 0.003.

Duroid appears on ebay but it is not cheap.

Kerry VK2TIL.
10343 2014-10-05 20:52:30 Jonathan Poland Re: A materials question - pcb as capacitor
I know I'm joining this discussion a little late.  Please forgive that.

I published an article in the Spring QRP Quarterly that discusses a variation of trombone capacitor for use with a 40-10m mag loop antenna.  Two of its unique features were the use of hobby store "telescopic" brass tubing for the plates and Dura-Lar brand mylar for the dielectric.  I think you should consider this type of design.

Benefits of Dura-Lar are:
- Extremely high breakdown voltage:  7000 volts/mil.  In my design, I used 10mil for a rated 70,000V breakdown voltage.  (No I did not test it but that is high enough to all full legal power to the mag loop if you wanted.)
- Good dielectric constant:  >3.2.  Because my dielectric included an air gap too, the measured dielectric was 2.4.  This is still respectable.
- Low cost.  Duralar is sold in sheets at art supply stores and on Amazon.  ( http://www.amazon.com/s?ie=UTF8&field-keywords=duralar&index=blended&link_code=qs&sourceid=Mozilla-search&tag=mozilla-20
10344 2014-10-06 00:41:36 radioaustralia123 Re: [emrfd] A materials question - pcb as capacitor
Hi Jon,

Any chance of getting a copy of your 40-10m magnetic loop antenna?

Regards,

Roderick Wall, vk3yc.

10351 2014-10-06 14:40:24 Chris Howard w0ep Re: A materials question - pcb as capacitor
I ended up buying some 1/8" lexan at the local big-box building supply
and some pretty stiff brash sheet at the hobby store.
I made about a 1 square inch capacitor clamped together with
nylon hardware.

My good old grid dip meter says it is resonant at about the
right place. I need to work on the feed arrangement,
but I am hopeful that I can make it work.

Thanks for the Dura-Lar info!



10353 2014-10-06 14:48:07 Dave Miller Re: A materials question - pcb as capacitor
Jon,
Which issue and what page? I can't seem to find it in any of my copies.  
Thanks
Dave

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